Prebiotics are generally sugars that are specifically chosen to not be absorbed by the gut, so that they can be food for the yeasts and microbes that live in your lower intestines.
Probiotics are doses of the yeasts and microbes - and again you don't want these to be absorbed by the gut - they need to live on the gut walls and feed on your stools.
But yes, I agree - research shows that only limited amounts of both of these will reach the lower intestine. BUT, once there they should multiply on the rich food source. With a small 5g bag of yeast, I can easily ferment a 25 litre bucket of wort to make beer (as long as no other bacteria or yeast get their first, that the wort isn't treated with something poisonous to yeast, that the temperature isn't too hot or too cold...)
Probiotics are doses of the yeasts and microbes - and again you don't want these to be absorbed by the gut - they need to live on the gut walls and feed on your stools.
But yes, I agree - research shows that only limited amounts of both of these will reach the lower intestine. BUT, once there they should multiply on the rich food source. With a small 5g bag of yeast, I can easily ferment a 25 litre bucket of wort to make beer (as long as no other bacteria or yeast get their first, that the wort isn't treated with something poisonous to yeast, that the temperature isn't too hot or too cold...)